""The water surface of your main pond should be at the same level of the patio, or higher; preferably higher. So the edge of the pond wall should be higher than the patio level. It needs to be at least a little higher than the patio level in order to keep the leaves and dusts out. ""

I was thinking the same. I might rised the pond water level up to around 2" over the patio pavement and my wall will be 6" above patio pavement level. The reason I want the box filter b/c this is the only filter that I know how to build and cheap. I've read and watched on youtube on how to built koi filter but I can't see how it works better than bog filter base on my limitted knowlege + they're so hard to hide and require weekly clean out ?. But anyway I am still do research on DIY filter.

The most important thing you have to decide is whether you want a regular garden pond for a few small koi or a true koi pond. With a regular garden pond, you can do whatever and it will be ok for a few fish. But a real koi pond with quality fish is much more technical, and will require a lot of research and planning and money and time and space.

The bog system depends a lot on plants. Most marginal plants , like water hyacinths and irises, etc., are seasonal plants. They die back in the winter, and your bog will look ugly. They are very pretty in the summer though, so you could have some around - outside of the pond to help softens up the edges. That's what I do. A good bog system requires a lot more surface area (a bigger foot print) than a conventional filtration tanks system. A good bog system requires a sand or gravel bed with some undergravel filter for drainage and cleaning , otherwise it will get stagnant and anoxic really quickly. Again, if your pond is only for a few little koi, then it wouldn't matter if the bog is bogged down or not. If you are really set on having a bog, then you could built it into your waterfall space for aesthetic reason, but you should not expect it to do the complete de-toxification of your pond water. You'd need more than that for a good koi pond.

Your first pond plan is a bit closer to the conventional koi pond setup. So with that, I sketched you a plan that is very generic and basic. Almost like what I have. See attached. It is self-explanatory. You basically have 2 tracks to filter water, one for the waste from the bottom drain and one for the oily scum on the surface. Here are some notes in case you are not familiar with the set up:

- The tanks (or chambers) could be made of concrete, liner, or plastic drums or just buy a complete set up if you have the money. You can do one big tank and divide it up by plexiglass plates or you can use plastic barrels and connect them by PVC pipes.
- Tank 1 is the settling chamber. You can be creative here and add many DIY projects, like stand pipes, airlift tubes to move waste to a basket for easy daily removal, or connect it to a Cetus sieve if you have money.
- Tank 2 is a brush or J-mat chamber.
- Tank 3 is another Japanese mat chamber.
- Tank 4 is from your surface skimmer. You have a lot of options here. This could be a Bakki shower, protein skimmer, or your bog with a waterfall. It needs a waste drainage, just like the other tanks.

Tanks 1-3 are below ground so you should cover it with a nice wood deck to stand on and enjoy your koi.
Tank 4 is above ground so you should try to camouflage it somehow.
The skimmer can be a simple overflow into a bog with marginal plants if your pond wall is high enough. But this could be tricky because you'd need to adjust the flow rates or timing of the pumps.
To save electricity, your Pump #2 does not have to run 24/7. It is basically your back-up in case your Pump #1 goes out.
The pumps should be below grade, but make sure there is cover and drainage of your pump chambers so they don't get flooded. (Houston rain too much!). They can both be below your deck.
You'd need an air pump. Use it to move water too and not just for oxygenation.
You might not need a UV if you need to save money.

Hi Tuan, the total volume of these filtration tanks depend on the pond volume. If you use the plastic barrels like those I showed, then it is modular and you can connect more barrels in series if needed. For a 2700 gal pond, 3 - 55 gal barrels (tank 1,2,3) here may be enough because you'd have other components. But a 55 gal settling tank (tank #1) is rather small, so you would need to design a way to clean out the waste more often. The old concept of letting the waste "settle" is changing. The wastes need to be removed from the water continously somehow.

A sand/bead filter is a good source for denitrifying bacteria and water polishing too, but you can replace it with another DIY barrel of J-mats or that sand barrel that Ricshaw recommended. Basically you need some filtration between the pump#1 and the waterfall. This last barrel will be above ground and will need waste drainage. Some people here use these barrels for their entire filtration system. Cheapest way and very effective, but very ugly.

I would also comment that connecting bio-filters in parallel is better than series.

Yeah, you're right about the parallel connection, but for just 3 or 4 barrels, there's no difference. A tall waterfall will hide a lot of ugliness. But most waterfalls are ugly too. It is hard to put a pile of rock in the middle of a flat yard and make it look believable.